1. Field of the Invention
The present invention relates to apparatuses for forming thin films on specimen substrates and apparatuses for extracting ions for forming thin films on specimen substrates, etching the surface of a thin film or improving the quality of the surface of a thin film, and more particularly a novel film forming apparatus capable of forming thin films of various materials at a high film growing rate with a high efficiency and stability for a long period of time by utilizing high-density plasma and a novel sputtering type ion source cable of extracting various ions of high current density with a high efficiency and stability for a long period of time. Furthermore, the present invention relates to a plasma generating apparatus which is capable of generating high-density plasma in a gas under a low pressure and which can be utilized with the film forming apparatus and the ion source.
2. Description of the Prior Art
In various types of LSI production processes, the techniques regarding to the formation of thin films and to ion sources presently occupy very important positions.
The sputtering apparatuses for forming thin films by sputtering targets in a plasma have been widely used for the formation of thin films of various materials and among them the so-called two-electrode (rf, dc) sputtering apparatus in which a target and a substrate are disposed in opposing relationship and are spaced apart from each other by a predetermined distance (for instance, as is disclosed by F. M. D'Heurle, Metallurgical Transactions, Vol. 1, (1970), pp.725-732) is most popular among those skilled in the art. Referring to FIG. 1, this apparatus comprises a vacuum chamber 4 in which are disposed a target 1 and a substrate 2 over the upper surface of which a thin film is grown, a gas introduction system 5 and an exhaust system 6 and a thin film is formed over the surface of the substrate by sputtering the target 1 by plasma 3 generated within the vacuum chamber 4.
In the cases of the conventional sputtering apparatuses, in order to increase the deposition rate of a thin film, plasma must inevitably be maintained in a high density state. In the case of the sputtering apparatus as shown in FIG. 1, the higher the density of plasma, the more rapidly the voltage applied to the target rises, so that the substrate is quickly heated and thin films formed are damaged by the impact due to the incidence of high-energy particles or (high-energy) electrons in the plasma. As a result, high-rate sputtering deposition can be carried out only with special heat-resisting substrates, materials and compositions of thin films.
Furthermore, in the conventional sputtering apparatuses, the discharge cannot be maintained in a stable manner in a low gas pressure range less than 10.sup.-3 Torr and plasma is generated only at a gas pressure of the order of 10.sup.-2 Torr or higher, so that there arises the problem that a large amount of impurities is penetrated into the thin film.
The particles which contribute to the growth of a thin film are almost neutral and it has been difficult to control the energy of such neutral particles.
Meanwhile, the magnetron sputtering apparatuses (for instance, as disclosed by R. K. Waits, J. Vac. Sci. Technol., Vol. 15 (1978), pp.179-187) and the facing targets sputtering apparatuses (for instance, as disclosed by M. Matsuoka et al., J. Appl. Phys., Vol. 60 (1986), pp.2096-2102) which permit a high-rate sputtering in a gas under a low pressure have been devised and demonstrated.
In the magnetron sputtering apparatuses, the high-energy secondary electrons are trapped over the surface of the target by the effects of the magnetic field closed over the surface of the target and the electric field over the surface cf the target so that high-density plasma can be generated in a gas at a low pressure. However, they have the problem that the qualities of the portions of a grown film corresponding to the eroded portions of the target to the not eroded portions, respectively, are widely different from each other. Furthermore, when the target is made of a magnetic material such as Fe, the magnetic flux does not leak to the surface of the target so that high-density plasma cannot be generated and the kinds of thin films to be formed are limited.
In the facing targets sputtering apparatuses, as shown in FIG. 2, the magnetic fields produced by permanent magnets 7 are applied between the targets 1 arranged in opposing relationship with each other so that the high-energy secondary electrons are confined between the targets to generate high-density plasma therebetween. They have the special feature that almost all kinds of thin films can be formed over the surface of the substrate 2 at a high deposition rate. The substrate 2 can be heated by a heater 8. In this apparatus, the impingement of the high-energy particles on the surface of the substrate is decreased so that this apparatus is regarded as one of the better apparatuses for forming a high quality thin film at low temperatures. However, the targets 1 are disposed in opposing relationship and are spaced apart from each other by a suitable distance, so that the substrate 2 must be located at a horizontal position and the deposition rate of the sputtered particles deposited over the surface of the substrate 2 is low. Furthermore, in the case of coating a large surface of a large-sized disc or the like, there arises the problem that the deposition rate or efficiency is essentially low when the targets are disposed in the manner described above.
Meanwhile, the ion sources which utilize an ion extracting mechanism such as a grid to extract the ions produced in plasma have been widely used for forming thin films of various materials, etching the surface of a formed thin film, processing the formed thin films and so on. Among them, the Kaufman type ion source provided with a filament for emitting thermal electrons so as to produce plasma (for instance, as disclosed by R. H. Kaufman et al., J. Vac. Sci. Technol., Vol. 21 (1982), pp.764-767) has been especially widely used. As shown in FIG. 3, in the Kaufman type ion source, disposed within the plasma generating chamber (vacuum chamber) 4 is a filament 9 for emitting thermal electrons. The filament 9 is used as a cathode so as to trigger the discharge in the magnetic field produced by an electromagnet 10 adapted to produce the magnetic field in order to stabilize plasma so that plasma 11 is produced. The ions in the plasma 11 are extracted by a plurality of ion extracting grids 12, as an ion beam 13.
The conventional ion sources which are typically represented by the Kaufman type ion source utilize the thermal electrons emitted from the filament to generate plasma so that the material of the filament itself is also sputtered and is included in the ions being extracted. Furthermore, when a reactive gas such as oxygen is used as a gas for generating plasma, it chemically reacts with the filament so that the ion extraction cannot be continued for a long period of time. In addition, the ion extraction is limited only to the use of gases such as Ar.
As the metal ion sources, the evaporation type ion sources and the sputtering type ion sources are well known to those skilled in the art. However, the evaporation type ion source must maintain the temperature within its furnace at high temperature, so that the vaporized particles are ionized and consequently impurities most frequently tend to be contained in a thin film being grown. Furthermore, the extraction of ions of a material having a high melting point is difficult (for instance, as disclosed by M. A. Hasan et al., J. Vac. Sci. Technol., Vol. B5 (1987), pp.1332-1339). In the cases of the sputtering type ion sources, metal ions obtained by sputtering a target in plasma are selectively extracted, but it is difficult to extract high-current ions over a large area (for instance, as reported by B. Gavin, IEEE Trans. Nucl. Sci., Vol. NS-23 (1976), pp.1008-1012).
In order to realize a high-current ion source by utilizing sputtering, the plasma density must be maintained at a high level with a high efficiency. To this end, the secondary electrons emitted from the target must be efficiently confined, but the conventional ion sources cannot satisfactorily confine the secondary electrons.
An ion extracting method with a high-efficiency and a large-area is disclosed by, for instance, N. Terada et al., Proc. Int'l Ion Engineering Congress, ISIAT'83 and IPAT'83, Kyoto (1983), pp.999-1004. According to this method, a negative potential is applied to a pair of opposing targets so that the high-energy secondary electrons are confined between the targets by the magnetic field produced therebetween. As a result, high-density plasma can be generated and the extraction of metal ions can be realized with a high efficiency. However, according to this method, the ion extracting holes are formed through the target so that the target itself has a function of a grid means for extracting ions. As a result, it is difficult to extract ions in a stable manner for a long period of time.
As described above, the conventional film forming methods cannot satisfy the following conditions simultaneously:
(a) A thin film is formed at a high deposition rate without causing damage to the thin film being grown and the substrate and an extreme temperature rise; PA0 (b) The energy of each particle incident on the substrate is low; PA0 (c) The ionization ratio of plasma must be maintained at a high value; PA0 (d) The discharge can be carried out in a gas under a low pressure; and PA0 (e) The efficiency of the deposition of atoms or ions sputtered from the target over the surface of the substrate must be high. PA0 (a) Ions with a significant current density can be extracted in a large area, hence the yield of the ion must be high; PA0 (b) The thin film formed must have a high purity; PA0 (c) The control of the ion energy must be carried out in a simple manner; PA0 (d) The extraction of almost all the ions including those of the materials having a high melting point can be carried out; PA0 (e) The ion production process must exclude a heating and vaporizing step; and PA0 (f) The ion extraction can be continued for a long period of time in a stable manner.
In like manner, the conventional sputtering type ion source techniques cannot satisfy the following conditions simultaneously: